This thesis presents an investigation into a wide range of potential materials
for organic photovoltaic (PV) devices. A variety of optical techniques
are used to define physical parameters for each material such
as the photoluminescence quantum yield (PLQY), absorption coefficient
and exciton diffusion length. Electrical characterisation is used to determine
the optimal structure for devices fabricated with these materials.
A number of novel materials are presented in this thesis. These include
new polymers, both soluble and precursor, and a relatively new class
of material, the conjugated dendrimer. These are highly configurable
branching molecular structures that enable fine tuning of material properties.
Work on polymers presented in this thesis investigates how such materials
can be improved by testing the effect of small changes to their
molecular structure. One of these changes had significant effects upon
the overall material characteristics. The introduction of a dipole across
a polymer successfully created a charge separating material without the
need for an extra species such as C60 to be present.
The introduction of the conjugated dendrimer to PV applications allows
significant scope for molecular engineering. Dendrimers enable tight
control over certain aspects of the molecular properties. Small changes
can be made such as colour tuning or solubility that enable optimisation
to be performed on the molecular level, rather than on device structure.
Such changes produced significantly higher internal quantum efficiencies
(> 90%) than typical polymer devices and offer the prospect of power
conversion efficiencies in excess of 10%.
Time-resolved luminescence (TRL) spectroscopy was used to characterise
the behaviour of photogenerated excitons within organic films. The investigation
of exciton diffusion length was performed upon two polymers,
each utilising two different time-resolved methods; diffusion to a quencher
and exciton-exciton annihilation. It was found that diffusion in
polythiophene films is anisotropic and the photoluminescence lifetime
is dependent upon film thickness. This is explained by the formation of
self-ordered microstructures during the spin coating process. Data modelling
was performed which took into account both the thickness variation
and the interaction of excitons with a quenching interface producing
a much more realistic approach than previously published work.